This is what we are going to make
Locking Transparency in Photoshop is something you must learn, it allows you to paint on a layer without affecting transparent areas. With this option you can change fill color without applying any blending option or filters or anything else.
Imagine you want to change fill color of this brush stroke (you can do this before you made the brush stroke, but what if once you have no option to undo or like that).
You can easily do this by layer blending options like color overlay but there is another easier way to do this.
just click on the Lock Transparent Pixels option in the layer pallet.
after that pick your brush and chose the color you want. Then start painting on it.
Just change the brush color to add any color you want